1. Field of the Invention
The present invention relates to a method of manufacturing a thin film transistor in which an amorphous silicon film is crystallized by using heat treatment with a laser beam, a thin film transistor obtained with the use of such crystallization, and a display unit including such a thin film transistor.
2. Description of the Related Art
In these years, as one of flat panel displays, an organic EL display unit that displays images by using organic EL (Electro Luminescence) phenomenon has been noted. The organic EL display unit has superior characteristics that the view angle is wide and the power consumption is small since the organic EL display unit uses light emitting phenomenon of organic light emitting device itself. Further, the organic EL display unit has high response to high-definition high-speed video signals, and thus the organic EL display unit is under development toward the practical use particularly in the image field.
Of driving methods in the organic EL display unit, active matrix method using a thin film transistor (TFT) as a driving device is more superior to passive matrix method in terms of responsibility and resolution. Therefore, the active matrix method is regarded as a particularly suitable driving method in the organic EL display unit having the foregoing advantages. The active matrix organic EL display unit has a driving panel in which an organic EL device including an organic light emitting layer and the driving device (the foregoing thin film transistor) for driving the organic EL display device are arranged. The driving panel and a sealing panel are bonded to each other with an adhesive layer in between, so that the organic EL device is sandwiched between the driving panel and the sealing panel.
It is known that in the thin film transistor as the driving device, when the voltage is continuously applied to the gate electrode, the threshold voltage of the transistor is shifted. When the threshold voltage of the thin film transistor is shifted, the current amount flowing into the transistor is changed. In the result, the luminance of the organic EL device composing each pixel is changed. However, some of the thin film transistors of the organic EL display unit need to maintain a conductive state as long as the organic EL device emits light. Therefore, it has been difficult to inhibit the shift phenomenon of the threshold voltage.
Therefore, in these days, to inhibit the shift phenomenon of the threshold voltage, the following organic EL display unit is developed. In the organic EL display unit, a channel region of the thin film transistor is made of a crystalline silicon film. The crystalline silicon film is obtained by irradiating an amorphous silicon (a-Si) layer formed on a glass substrate with an excimer laser beam to provide annealing treatment.
However, when the crystalline silicon film is formed by annealing treatment with the use of the excimer laser beam, there is the following shortcoming. That is, when heat distribution is caused by energy distribution in the irradiation area or energy variation among pulses of the laser beam, crystallinity of the crystalline silicon film becomes uneven in the crystal face. Thus, characteristics of the thin film transistor vary.
Therefore, for example, in Japanese Unexamined Patent Application Publication Nos. 60-18913, 4-332120 and 2002-93702, and Japanese Examined Patent Application Publication No. 3-34647, the following technique is proposed. In the technique, a photothermal conversion layer is formed on an amorphous silicon film, the photothermal conversion layer is irradiated with an excimer laser beam, and thereby the amorphous silicon film is crystallized. According to the technique, the calories necessary for crystallizing the amorphous silicon film can be indirectly provided by using the photothermal conversion. Therefore, uniform crystallization in the crystalline silicon film is realized, and in the result, a thin film transistor having the uniform electric characteristics can be obtained.
More specifically, in the method not using the photothermal conversion layer, the amorphous silicon film is directly irradiated with a laser beam and crystallized by using laser having the absorption wavelength corresponding to the silicon film. In this case, however, for example, in the large substrate being 300 mm to 920 mm in one side, the film thickness variation of about 10% is generated. Therefore, when the amorphous silicon film is thinned, the absorption ratio of the laser beam is lowered, leading to crystallinity variation. Meanwhile, in the method using the photothermal conversion layer, as long as the surface reflectance and the film thickness of the photothermal conversion layer are uniform, it is possible to realize uniform crystallization independently of the wavelength of the laser beam and the film thickness of the silicon film.